It is previously known to permanently anchor oral and extraoral prostheses in bone tissue. In order to avoid subsequent loosening of the prosthesis, a healing period is required during which there is a direct contact, i.e. an exact adaption, between the prosthesis and the bone tissue. Such a permanent, exact adaption which allows loading is called osseointegration. The possibility of such a long-term adaption and function in clinical practice of load-supporting artificial anchoring members was first indicated by Dr. Branemark et al. They showed that screw-shaped anchoring members, so-called fixtures, of pure titanium were osseointegrated if a number of prerequisites were fulfilled. The osseo-integration principle developed by Professor Branemark et al has been used in clinical practice for 20 years with good results and has been described for instance in:
Branemark/Zarb/Albrektsson: "Tissue-Integrated Prostheses, Quintessence Books, 1985.
The osseointegration principle is based not only on the use of an implant made of pure titanium but also on a specific operating technique which comprises installation of the titanium fixture in a first operation with a minimum of surgical trauma, a healing period for a specific period of time and without any direct load on the fixture, and a second operation in which the prosthesis part (abutment) is attached to the fixture, i.e. a two-stage operation with an intervening, unloaded healing period.
In order to achieve a good healing of the fixture within the bone tissue, a minimum of negative biological reactions should be induced, or even better, positive reactions only. The structure and the chemical composition of the fixture surface has been of a significant importance for these reactions.
In Swedish patent 7902035-0 it is illustrated how improved results can be achieved by a specific surface structure of the titanium material of the fixture. The surface layer of the fixture comprises a micro-pitted titanium dioxide in which the surface pits have a diameter of 10-1000 nm, preferably 10-300 nm. In Swedish patent application 8505158-9 is described a chemical composition for the titanium surface layer which has been made of specific advantage for the tissue reactions.
It is also important that the fixture has an adequate geometrical design in order to facilitate the requirement of an atraumatic surgical operation as well as a surface cleanliness. Swedish Patent specification 332 486 describes a bolt-shaped anchoring member with an external thread. The threaded part of the bolt is intended to be inserted into a hole drilled in the jaw bone and the bolt is provided with an internal thread for a permanent implantation of a dental anchoring member. The bolt is provided with a bore which is open at its forward end and provided with radially directed openings through the threads into the bore. The bolt also has a conical forward end part. The openings at the forward end part of the bolt are intended to facilitate the healing of the bolt into the jaw bone through in-growth of newly formed bone tissue through the openings to prevent the bolt from being screwed out.
As already mentioned the bolt is intended to be inserted into a hole drilled in the jaw bone. To satisfy the requirement of an atraumatic surgical operation technique, the preparation of the bone hole is made in several steps using drills with successively increasing drilling diameters as well as a series of screw taps. In order to achieve an atraumatic preparation of the fixture site, it is required that the temperature is maintained low, which involves low drilling speeds and adequate cooling by means of profuse irrigation with saline solution. The surface of the fixture must be very clean, which requires a careful cleaning after every operation step or the use of non-returnable instruments.
In order to facilitate the surgical method and decrease the number of instruments required for preparation of the bone tissue, it is previously known to provide the fixture with cutting edges for self-tapping when the fixture is installed in the bone tissue. In addition to forming the bone in-growth openings, a further machining of the end part of the fixture is therefore required in order to form such cutting edges. Examples of such self-tapping fixtures can be found in EP-A-139052 and DE-A-3043336.
The formation of the above-mentioned openings and cutting edges results in a complicated and expensive manufacturing method, often leaving burrs which are difficult to remove on inner surfaces. The cutting edges which have been extended to the forward end surface of the fixture may also damage the bone membrane during use.